Characterization of the Interaction of Bacillus subtilis PyrR with pyr mRNA by Site-Directed Mutagenesis of the Protein

The Bacillus subtilis PyrR protein regulates transcriptional attenuation of the pyrimidine nucleotide ( pyr ) operon by binding in a uridine nucleotide-dependent manner to specific sites on pyr mRNA and stabilizing a secondary structure of the downstream RNA that favors termination of transcription. The high-resolution structure of unliganded PyrR was used to guide site-directed mutagenesis of 12 amino acid residues that were thought likely to be involved in the binding of RNA. Missense mutations were constructed and evaluated for their effects on regulation of pyr genes in vivo and their uracil phosphoribosyltransferase activity, which is catalyzed by wild-type PyrR. A substantial fraction of the mutant PyrR proteins did not have native structures, but eight PyrR mutants were purified and characterized physically, for their uracil phosphoribosyltransferase activity and for their ability to bind pyr RNA in vitro. On the basis of these studies Thr-18, His-22, Arg-141, and Arg-146 were implicated in RNA binding. Arg-27 and Lys-152 were also likely to be involved in RNA binding, but Gln substitution mutations in these residues may have altered their subunit-subunit interactions slightly. Arg-19 was implicated in pyr regulation, but a specific role in RNA binding could not be demonstrated because the R19Q mutant protein could not be purified in native form. The results confirm a role in RNA binding of a positively charged face of PyrR previously identified from the crystallographic structure. The RNA binding residues lie in two sequence segments that are conserved in PyrR proteins from many species.